The invention relates to an apparatus for detecting amount of leftover lower thread which may be utilized in an industrial sewing machine, for example.
A prior art for an apparatus for detecting amount of leftover lower thread of the kind described is disclosed in Japanese Utility Model Publication No. 26,455/1990, for example.
In the prior art practice, an apparatus for detecting amount of leftover lower thread in a sewing machine comprises an optical detector located adjacent to a bobbin on which a lower thread is disposed as a winding for detecting the level of transmitting or reflected light which varies with the amount of winding of lower thread on the bobbin or the leftover amount thereof, thus directly deriving the amount of leftover lower thread.
However, the use of an optical detector of this kind requires a bobbin or a shuttle race which is machined to a special configuration, and a customary bobbin or shuttle race cannot be used with such detector. An optical detector is susceptible to a malfunctioning caused by a contamination thereof caused by fragments of thread or by oil which is unavoidable in an industrial sewing machine. Since an industrial sewing machine is designed for continuous operation over a prolonged period of time under nearly unmanned condition, the detector may fail to detect the exhaustion of the lower thread as a result of a malfunctioning thereof to allow a continued operation of the machine. Then there results a number of unsewn fabrics, requiring a repeated sewing operation and thus degrading the production efficiency.